1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically-operated vehicular transmission and relates, in particular, to an apparatus in which in-gear shocks at the time of gear engagement can be reduced.
2. Description of the Related Art
As a method of controlling an oil pressure in a hydraulic engaging element of a hydraulically-operated vehicular transmission, there are the following two methods. One is a direct control method in which the oil pressure in the hydraulic engaging element is directly controlled by a pressure regulating solenoid valve. The other is an accumulator back pressure control method in which a back pressure in the accumulator connected to an oil passage communicated with the hydraulic engaging element is made variable depending on an engine load so that the oil pressure increase in the hydraulic engaging element is buffered in an oil pressure region corresponding to the engine load.
In the above-described direct control method, the oil pressure in the hydraulic engaging element should ordinarily be controlled at a high accuracy through the pressure regulating solenoid valve by a command value from en electronic control unit. Actually, however, there are variations in the performance of the pressure regulating solenoid valve and an output performance (or output characteristics) of the electronic control unit because they are mass-produced. As a consequence, at the time of gearing in with the throttle valve fully closed, at which a delicate oil pressure control is required in a low-pressure region, the oil pressure control in the hydraulic engaging element sometimes goes wrong, resulting in shocks.
In the above-described accumulator back pressure control method, on the other hand, there is the following disadvantage. Namely, at the time of gearing in with the throttle valve fully closed, if an accelerator is depressed right before the completion of pressure accumulation in the accumulator, the oil pressure in the hydraulic engaging element increases without being subject to the pressure increase buffering function in an oil pressure region depending on the engine load, resulting in shocks.
In view of the above-described points, the present invention has an object of providing a control apparatus for a hydraulically-operated vehicular transmission in which the accumulator back pressure control method and the direct control method are combined so that the in-gear shocks can be buffered under any driving conditions of the vehicle.
In order to attain the above and other objects, the present invention is a control apparatus for a hydraulically-operated vehicular transmission comprising: a pressure regulating solenoid valve for controlling a pressure in a hydraulic engaging element which establishes a predetermined speed transmission train of the hydraulically-operated vehicular transmission; an accumulator connected to an oil passage which is communicated with the hydraulic engaging element; discriminating means for discriminating whether a pressure accumulation in the accumulator has been completed or not; control means for maintaining an outlet pressure of the pressure regulating solenoid valve at a predetermined pressure until the pressure accumulation in the accumulator has been completed and for increasing the output pressure of the pressure regulating solenoid valve, after the pressure accumulation in the accumulator has been completed, depending on a driving condition of the vehicle.
In the present invention, what the pressure regulating solenoid valve shares is the pressure increase in the hydraulic engaging element after the completion of pressure accumulation in the accumulator. Even if the accuracy in the pressure control in the low-pressure region becomes poor by variations, due to mass production, in the performance of the pressure regulating solenoid valve or the output performance in the electronic control circuit, the oil pressure increase in the hydraulic engaging element in the low-pressure region is buffered by the accumulator. Therefore, the occurrence of in-gear shocks with the throttle valve fully closed can surely be buffered.
Further, at the time of gearing in with the throttle valve fully closed, even if the accelerator is depressed right before the completion of pressure accumulation in the accumulator, the in-gear shocks can surely be buffered as a result of control, by the pressure regulating solenoid valve, of the oil pressure in the hydraulic engaging element after the completion of accumulation.
The following arrangement is also conceivable. Namely, a sensor for detecting a stroke of a piston in the accumulator is provided. When the piston is at a full stroke, a discrimination or judgement is made that the pressure accumulation in the accumulator has been completed. This arrangement, however, requires a stroke sensor and becomes high in cost. Here, the hydraulic engaging element transfers, during the pressure accumulation of the accumulator, from an open state to an engaged state. Therefore, when the degree of slipping of the hydraulic engaging element has lowered to a predetermined value, a judgement can be made that the pressure accumulation in the accumulator has been completed. The degree of slipping of the hydraulic engaging element can be calculated from the rotational speed of the input shaft and the rotational speed of the output shaft of the transmission. In addition, these rotational speeds can be detected by rotational speed sensors which are originally provided in the transmission. Therefore, there is no need of providing a special sensor for discriminating the completion of pressure accumulation in the accumulator. The cost can thus be made small.
In order to improve the response of gearing in, it is desired, during pressure accumulation in the accumulator, to maintain the output pressure of the pressure regulating solenoid valve at a predetermined pressure which is higher than the oil pressure in the hydraulic engaging element at the time of completion of pressure accumulation, whereby the time required to the completion of pressure accumulation is prevented from being prolonged. However, if the output pressure of the pressure regulating solenoid valve is maintained at this kind of predetermined pressure until the completion of pressure accumulation in the accumulator, the oil pressure in the hydraulic engaging element will rapidly rise from the oil pressure at the time of completion of pressure accumulation to the predetermined pressure, resulting in the occurrence of shocks. In such a case, the following arrangement may be made. Namely, as a value to discriminate the degree of slipping in the hydraulic engaging element, there are set a predetermined first value which corresponds to the state of under pressure accumulation and a predetermined second value which corresponds to the state of completion of pressure accumulation. Until the degree of slipping falls to the first predetermined value, the output pressure in the pressure regulating solenoid valve is maintained at a predetermined value. Until the degree of slipping lowers to the predetermined second value after it has lowered below the predetermined first value, the output pressure of the pressure regulating solenoid valve is gradually decreased from the above-described predetermined pressure.
Once the slipping degree has fallen below the predetermined second value, the output pressure of the pressure regulating solenoid valve is increased at a pressure increase characteristic depending on the driving condition of the vehicle. According to this arrangement, the output pressure of the pressure regulating solenoid valve is gradually decreased after the slipping degree has fallen below the predetermined first value. Therefore, when the slipping degree has fallen to the predetermined second value, the output pressure of the pressure regulating solenoid valve is made to coincide with the oil pressure in the hydraulic engaging element at the time of completion of pressure accumulation. The occurrence of shocks due to an increase in the oil pressure right after the completion of pressure accumulation can thus be prevented.